Antihunt electrical control system



May 25, 1948. 2,442,329

T. RI HARRISON ET AL ANTIHUNT ELECTRICAL CONTROL SYSTEM Filed Feb. 7, 1944 3 Sheets-Sheet 1 FIG.!

INVENTOR. THOMAS R HARRISON LLOYD B. CHERRY y 1948- TQR. HARRISON ETAL 2,442,329

ANTIHUNT ELECTRICAL CONTROL SYSTEM Filed Feb. 7, 1944 3 Sheets-Sheet 2 INVENTOR. THOMAS R HARRISON LY .HRF-ZY BY LODBCE ATTOR Y.

May 25, 1948.

T. R. HARRISON ETAL 2,442,329 ANTIHUNT ELECTRICAL CONTROL SYSTEM Filed Feb. 7, 1944 3 She ets-Sheet 3 ll'li'llli.

Illll IlllO INVEMTOR. THOMAS R. HARRISON LLOYD B.CHERRY ATTORNEY.

Patented May 25, 1948 I 2,442,329 ANTIHUNT ELECTRICAL CONTROL SYSTEM Wyncote, and Lloyd B.

Thomas R. Harrison,

Cherry, Philadelphia Brown Instrument Co a corporation of Pennsylvani Pa., assignors to The il Philadelphia, Pa., a

Application February 7, 1944, Serial No. 521,336

16 Claims.

The present invention relates to improvements in automatic exhibiting and/or controlling systems for eliminating the hunting tendency in such systems.

A specific object of the present invention is to provide self-balancing exhibiting and/or controlling apparatus with novel and effective means for including an anticipating control action in the apparatus rebalancing operations so as to eliminate or substantially minimize the hunting tendency which inevitably results from failure to terminate the rebalancing operations promptly enough to avoid over-adjustment of the rebalancing components of the apparatus.

The present invention is of especial utility in sell-balancing electrical networks of the potentiometer type in which rebalancing operations are rapidly eflected as soon as required by means comprising a reversible rotatable rebalancing motor. Such a motor has a tendency to coast" or continue to rotate after being deenergized and thus has a tendency to continue its rebalancing adjustment of the electrical network after the motor has been deenergized.

The need for anti-hunting" provisions in recording and/or controlling apparatus has long been recognized and various arrangements have been previously proposed for the elimination of hunting. One prior art method for preventing hunting is disclosed in patent Reissue 21,309 issued to Thomas R. Harrison on December 26, 1939, for Recording and control system and apparatus therefor in which the speed of rebalance of an electrical network of the above mentioned potentiometer type is efiected in accordance with the extent of network unbalance and mechanically movable means are provided for anticipating the true balance. The Harrison Patent 2,263,497 issued November 18, 1941, discloses a self-balancing potentiometric network including means for deriving an electromotive force proportional to the speed of the potentiometric rebalancing motor which is introduced into the detector circuit in opposition to the unbalanced electromotive force of the condition responsive network and operates to produce a condition of simulated rebalance of the potentiometric network before the latter is rebalanced to thereby anticipate the true balance position of the network. In this manner, the rebalancing driving system is slowed down before the true balanced position is reached and then gradually eases into said balance position without the occurrence of overshooting.

The said Harrison Patent 2,263,497 discloses a system including an electrical network and means for producing an unbalanced electromotive force in said network in accordance with the variations in magnitude of a condition under measurement and also includes a device which is adjustable to reduce the unbalanced electromotive iorce to zero. The said device is adjustable by .a reversible motor having a winding which is selectively energized by a detector responsive to the unbalanced electromotive iorce produced by the network to eifect rotation oi. the motor in one direction or the other in accordance with the sense of the said unbalanced electromotive force. In order to eliminate the hunting tendency oi the reversible motor, a bridge network including the said motor winding in one arm is provided and so arranged that an electromotive force proportional to the motor speed is derived at the output terminals oi, the bridge network upon motor rota tion. This last mentioned electromotive force is connected in opposition to the unbalanced electromotive force produced in the electrical network and operates by producing a simulated condition of rebalance of the network before the true balanced position is reached to prevent coasting and consequent over-shooting of the motor beyond the true balanced position.

A particular object of the present invention is to provide improvements in anti-hunting means of the type disclosed in the Harrison Patent 2,263,497, said improvements providing the desirable characteristics of simplicity and a high degree of stability.

The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects attained'with its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a diagrammatic illustration of a selfbalaneing potentiometric network utilizing the invention;

Fig. 2 is a wiring diagram showing only a portion of the electronic amplifier and motor drive circuit of the arrangement of Fig. 1 for the purpose of facilitating understanding of the invention; and

Figs. 3, 4, 5 and 6 illustrate different modifications of the arrangement of Figs. 1 and 2.

In Fig. 1 oi the drawings, we have illustrated,

3 more or less diagrammatically, an arrangement including an electronic amplifier device to be described for producing effects in accordance with the extent of unbalance of an electrical network or the potentiometric network type which controls the electronic device and is unbalanced in accordance with the variations in a quantity to be measured and in which it is not practicable nor desirable to have the said efieots directly produced by the potentiometric network perform a control operation because of the small network disclosed in the Harrison et al. Patent 2,150,502 issued on March 14, 1939.

The potentiometric network 4 is of a wellknown type, audit is believed to be suflicient for the present purposes to note that the potentiometric network includes a circuit branch in which theftliermocouple I is connected, an opposing branch including a source of known potential such as a battery 5 and resistors 5 and I, a variable portion of i which may be .connected'into the opposed branches in accordance with the adjustmentof'a sliding contact 8 whereby the respective eifects of the variable and known sources are made equal and opposite andv the potentiometric network is thus 'rebalanced for a given value of the electromotive force of the thermocouple I with the contact 8 in a corresponding position'alongthe length of resistances 6 and I. The position of the contact. 8 thus provides a measure of the value of the thermocouple electromotive force and may serve as a measure of the temperature to which the thermocouple is exposed.

Upon change in the temperature towhich the thermocouple I is subjected, an unbalanced direct current potential of one polarity or of the opposite'polarit'y. isproduced in the potentiometric network 4 depending upon the sense of unbalance of the potentiometric network 4 and consequently upon the direction of the temperature change. The unbalanced direct current potential so produced is impressed on the input circuit'of an electronic device indicated generally by the reference character 9. The electronic device 9 includes a vibrator or equivalent device 10, an input transformer I I, amplifying vacuum tubes l2 and I3, and a motor drive vacuum tube M.

The direct current potential applied to the input circuit of the electronic device 9 is translated by the vibrator l0 into a pulsating current having one polarity or the opposite polarity depending upon the sense of unbalance of the potentiometric network. This pulsating current is converted into an alternating current which is impressed on and amplified by the transformer H and is further amplified by the vacuum tubes i2 and I3. The output from the vacuum tubes I2 and I3 is impressed on the input circuit of the motor drive vacuum tube l4 and operates to control the conductivity of the latter as required to eflect selective operation of a reversible rotatable electrical motor 15 for rotation in one direc- 4 tion or the other. The motor i5 is employed to operate a slidewire assembly to rebalance the potentiometric network 4 and also serves to adjust an indicating and recording mechanism which is described hereinafter.

The potentiometric network 4 includesthree resistances l5, l1 and I8 connected in series in one branch. These resistances are formed of material having substantially a zero temperature coefiicient of resistance and are employed for calibration purposes. The battery 5, which may take the form of a dry cell, and a, dual vernier rheostat comprising resistances l9 and 20 and electrically connected sliding contacts 2| and 22 which engage the resistances l9 and 20, respectively, are connected in series in a branch in parallel to the branch including resistances l6, l1 and '18. The rheostat may be operated by any suitable type of knob, not shown, which desirably has a direct mechanical connection with the contact 2! and a lost motion connection with the contact 22. Upon initial movement of the knob the contact 2| is first moved and then contact 22 is moved thereby providing a vernier adjustment. To this end, the resistance 20 is preferably of higher resistance than the resistance 19.

Also connected in parallel with the resistances l6, l1 and I8 is a third branch including two series connected resistances 23 and 24. The resistance 23 is preferably made of nickel, copper or other material having a positive temperature coeflicient of resistance and the resistance 24 is formed of manganin having a substantially zero temperature coefiicient" of resistance. "The resistance 23 in conjunction with the resistance 24 operates to compensate for changes in the ambient temperature to which thecoldor reference junction of the thermocouple is subjected. The resistance is also provided for standardization purposes and has a resistance value such that the potential drop produced across it is of the same magnitude as the potential produced by a standard cell which is not shown but which may be periodically connected to the potentioinetric network! for standardizing the latter inthe manner disclosed, for example, in the Walter P. Wills application filed December 1, 1941, and having Serial No. 421,173, which issued as Patent No. 2,423,540 on July 8, 1947.

The slidewire assembly of the potentiometric network 4 consists of the resistances 6 and l and'the contact 8. The resistance 5 comprises a coil which is wound around and is insulated from a core 25. Cooperating with the slidewire 6 is the resistance 1 which constitutes a collector bar and comprises a coil wound around a core 26. The slidewire 6 and collector bar I are electrically connected by the sliding contact 8 which is moved by the reversible motor l5 as required to restore and maintain the potentiometric network 4 balanced. The terminals of the slidewire 6 and its core 25 are connected in parallel to the resistance IT.

The shaft of motor 15 drives a pinion 21 which is disposed in engagement with a gear 28. Attached to and movable with the gear 28 is a pulley 29 around which is wound an endless cable 30. The cable 30 is connected to the potentiometric rebalancing contact 8 so that when the motor i5 rotates the contact will be moved in one direction or the other to rebalance the potentiometric network. One end of the cable 30 runs over a pulley 3| which is pivotally mounted and biased by a spring 32 to take up the slack in the cable. The other end of the a stationary pulley 38.

A pen 34 is mounted on the carriage which carries the potentiometer rebalancing contact 8 and is arranged in cooperativerelation with a recorder chart 35 to thereby provide a continuous record of the adjustments of the potentiometer contact 8 which are required to maintain the potentiometric network 4 balanced, and accordingly, to provide a record of the variations in magnitude of the unknown potential produced by the thermocouple I. The chart 95 may be a strip chart as shown and is adapted to be driven in any convenient manner, as for example, by a unidirectional motor 38 through suitable gearing, not shown, so that a record of the variations in the unknown potential will be recorded as a continuous line on the chart 85.

The electronic device 8 is connected to and receives energizing current from the alternating current supply mains 81 and 38 through a pair of conductors 99 and 48. A double pole-single throw switch 4| is provided between electronic device 9 and the supply mains 81 and 88 for disconnecting the electronic device from the supply mains when it is so desired. Preferably, the switch 4I is so located that when it is adjusted to the position to deenergize electronic device 9 it also deenergizes the chart motor 88.

One input terminal of the electronic device 9 comprises the point of engagement of a pair of primary windings 42 and 48 provided on the transformer II. This input terminal is that to which one terminal of the thermocouple I is connected by the conductor 8. The primary windings 42 and 48 are wound around a core struc ture 44 on which is also wound a secondary winding 48. A shield 48 is provided between the primary windings 42 and 49 and the secondary winding 45. The windings 42, 49 and and the core structure 44 and shield 48 are all housed in a casing indicated generally by the reference character I IA.

The vibrator III may desirably be of the type disclosed and claimed in the copending application of Frederick W. Side filed December 1, 1941 and bearing Serial No. 421,176, and now Patent No. 2,423,524 of July 8, 1947, and comprises a vibrating reed 41 for operating a contact 50 with respect to a pair of oppositely disposed and relatively stationary contacts 5| and 52. The contact 58 is connected through the vibrating reed 41 to the potentiometric network point 49 through the conductor 48 and constitutes the second input terminal of the electronic device 9. The vibrating reed 41 is vibrated under the influence of a winding 53 which is supplied with alternating current from the secondary winding 54 of a transformer 55 having a line voltage primary winding 58 and additional secondary windings 51 and 58. A permanent magnet 59 associated with the vibrating reed 41 is utilized for polarizing and synchronizing purposes. The mounting structure for the vibrating reed contacts, operating windins. and permanent magnet are all connected to a grounded conductor located in the electronic device 9. A housing designated by the reference character I DA encloses the mechanism of the vibrator.

As illustrated in the drawing, the contact 5| of the vibrator I0 is connected by a conductor 8| to the terminal of the transformer primary winding 42 which is remote from the primary winding 43. Similarly, the contact 82 of the vibrator is connected by a conductor 82 to the terminal of cable runs around transformer primary winding 43 which is remote from the primary winding 42. As the vibrating reed 4I vibrates, therefore, the transformer primary windings 42 and 49 will alternately be connected in a series circuit which may be traced from one terminal of the thermocouple I through conductor 3, one or the other of the transformer primary windings 42 and 48, the vibrating reed 4I, conductor 48 to the potentiometric network point 49, contact 8 of the potentiometric network slidewire assembly, collector bar 1 and conductor 2 back to the other terminal of the thermocouple I. For convenience, the point of engagement of the contact 8 and the slidewire resistance 8 has been designated by the reference numeral 89.

With the arrangement thus far described, the magnitude and direction of current flow through the circuit branch from the potentiometric network point 49 to the vibrator I8, the transformer I I, and the thermocouple I to the potentiometric network point 88 depends upon the relation between the electromotive force produced by the thermocouple I and the potential diiference between the potentiometric network point 49 and the potentiometric network point 88. The thermocouple I is so connected to the potentiometric circuit that the electromotive force of the thermocouple I opposes the potential difference between the potentiometric network points 49 and 89. The potential difference between the potentiometric network points 49 and 89 is increased and decreased by movement of the sliding contact 8 to the right and to the left. respectively. With a suitable adjustment of the sliding contact 8, the potential difference between the potentiometric network points 49 and 83 will be made equal and opposite to the electromotive force produced by the thermocouple I and no current will flow through the above traced circuit including the vibrator I8 and the primary windings of the transformer II. On an increase in the thermocouple electromotive force above the potential difference between the points 49 and 89, current will flow in one direction through the vibrator I0 and the primary windings 42 and 48 of the transformer II and such current flow may then be eliminated by a suitable adjustment of the sliding contact 8 to the right When the electromotive force of the thermocouple I becomes less than the potential difference between the potentiometric network points 49 and 83, the current flow through the vibrator I8 and the primary windings of transformer II will be in such a direction as to be eliminated by a suitable adjustment of the sliding contact 8 to the left.

As noted above, the sliding contact 8 is adjustcd along the slidewire resistance 8 and the collector bar I by the reversible motor l5. The reversible motor I5 has a pair of terminals 84 and 85 which are' connected in the output circuits of the vacuum tube I4 and also has a pair of terminals 88 and 81 which are connected by means of conductors 88 and 89 to the alternating current supply main: 3! and 88 through the switch 4 I. A condenser 10 of suitable value is connected in the conductor 89.

The motor I5 comprises a rotor II and two pairs of oppositely disposed field poles on one pair of which a winding I2 is wound and on the other pair of which a winding 13 is wound. Winding I2 has its terminals connected to the motor terminals 88 and 81 and is supplied with energizing current from the alternating current supply mains 91 and 38 through the condenser 18. Due

'tion to rebalance the potentiometric network 4.

to the action of the condenser 18 the current which flows through the motor winding 12 will be in phase with the voltage of the alternatin current supply mains. The winding 13 has its terminals connected to the motor terminals 84 and 65 and is supplied with energizing current from the output circuits of the vacuum tube l4 which as shown includes two triodes. The current supplied to the winding 18 from the vacuum tube 14 either leads or lags by approximately 90 the voltage of the alternating current supply mains and establishes a field in the motor rotor 1| which is displaced 90 in one direction or the other with respect to that established therein by the winding 12. The reaction between the field set up by the winding 13 with that set up by the winding 12 establishes a rotating field in the rotor which rotates in one direction or the other depending upon whether the winding 13 is energized with current which leads or lags the voltage suppl ed by the supply mains 31 and 88 and consequently, as is explained in detail hereinafter, in accordance with the direction of unbalance of the potentiometric network 4. The direction and duration of rotation of the motor I is controlled in accordance with the direction and extent of unbalance oi the potentiometric network 4 sothat on rotation of the motor i5 the sliding contact 8 is adjusted in the proper direc- The amplifying tube l2 of the electronic device 9 includes two heater type triodes designated by the reference characters 14 and 15 within the same envelope. The triodes 14 and 15 each include anode, control electrode, cathode and heater filament elements.

gizing current from the transformer secondary winding 54. In order not to complicate the draw ing the conductors connecting the transformer connected in parallel; The triode 11 is utilized as a half wave rectifier to provide a source of unidirectional voltage for energizing the anode or output circuits of the triodes 14, I5 and 19. To this end, the control electrode and cathode of the triode 11 are directly connected to each other, and the output circuit thereof is energized by the transformer secondary winding 51 through a circuit which may be traced from the left end terminal of the winding 51 through aconductor 18 to the anode of triode 11, the cathode and through a conductor 19 to the positive terminal 88 of a filter generally designated by the reference character 8|. The negative terminal 82 of the filter is connected by a conductor 83 to the right end terminal of the transformer secondary winding 51 and is also directly connected to the grounded conductor 68.

The filter 81 includes a condenser 84 which operates to smooth out the ripple in the output voltage of the filter between the points 88 and 82 and also includes a resistance 85 and a con- The heater filaments areconnected in parallel and are supplied with ener- 8 denser 88 which operate to smooth out the voltage between the filter points 81 and 82. A further resistance 88 and a condenser 89 are provided in the filter to smooth out the voltage between the filter points 98 and 82. Accordingly, the filter comprises three stages. A three-stage filter is provided because for the most satisfactory and emcient operation it is desirable that the anode voltage supplied to the triode 14 be substantially free from ripple whereas it is not necessary to supply anode voltage so completely free from ripple to the output circuit of the triode 15. Likewise, it is not necessary to supply anode voltage so free from ripple to the triode 18 as it is to triode 15.

The anode or output circuit of the triode 14 may be traced from the filter point 98 which comprises the positive terminal of the direct current voltage supply through a fixed resistance 9| to the anode of triode 14, the cathode and through a cathode biasing resistance 92 shunted by a condenser 93 to the negative filter point 82 through the grounded conductor 88. The cathode biasing resistance 92 and condenser 93 are provided for biasing the control electrode of triode 14 negatively with respect to the cathode.

The input circuit of the triode 14 may be traced from the cathode through the parallel connected resistance 92 and condenser 93 and through the grounded conductor 68, transformer secondary winding 45 and a conductor 94 to the control electrode of triode 14. A tuning condenser 95' is preferably connected in shunt to the transformer winding 45 as shown. Y

The output circuit of triode 14 is resistancecapacity coupled to the input circuit of triode 15 by means of a condenser 95 and a resistance 98. Specifically, the anode of triode 14 is connected by condenser 95 to the control electrode of triode 15 and the control electrode {of triode 15 is connected through the resistance 96 to the grounded conductor 68 and thereby'to the cathode of triode 15 since .the latter is directly connected to the grounded conductor 88. o

The anode or output circuit of the triode 15 may be traced from the positive terminal 81 of the filter 8| through a fixed resistance 91 to the anode of triode 15, the cathode and the grounded conductor 88 to the negative terminal 82 of the filter.

The output circuit of triode 15 is resistancecapacity coupled to the input circuit of the triode 16 by means of a condenser 98 which is connected between the anode of triode 15 and the control electrode of the triodelfi and by means of a potentiometer resistance 99 which is connected between the control electrode of triode 16 and the cathode thereof. A contact I88 in adjustable eng-agement with the resistance 99 is provided for varying the point of connection of the control electrodeof triode 16 to the resistance 99. The resistance 99 and contact I88 perform a dual function, namely, to limit the extent to which the control electrode of the triode 16 may be driven positive with respect to its associated cathode and also to varythe proportion of the signal impressed on the control electrode of triode 16 from the output circuit of the triode 15. The resistance 98 connected in the input circuit of the triode 15 also serves to limit the extent to which the control electrode of triode 15 may go posiengagement with the reis provided for a purpose described sliding contact III in sistance ll hereinafter.

The electronic tube I4 includes two heater type triodes I02 and I03 within the same envelope. Anode voltage-is supplied the output circuit of the triode I02 from the transformer secondary winding 40 through a circuit which may be traced from the left end terminal oi the secondar winding 50 to the anode of triode I02, the cathode thereof to one input terminal I04 of a bridge network designated generally by the reference character I05 and from the other input terminal I of the bridge network through a conductor I01 to a center tap on the transformer secondary winding 50. The anode circuit of the triode I03 may be traced from the right end terminal of the transformer secondary winding 50 to the anode of triode I03, the cathode and through the bridge network I and conductor I01 to the center tap on winding 50.

As will be more readily seen by reference to Fig. 2 which discloses in a more schematic manner a portion of the electronic device 9, the wind- 13 of the reversible electric motor I5 comprises one arm of the bridge network I05. A diametricallyopposed arm of the bridge network includes a resistance I08 and a condenser I00 connected in series. The remaining two arms of the bridge network include fixed resistances IIII and III, respectively. A condenser II2 of value suitable for tuning the motor winding 13 is connected across the input terminals I04 and I05 of the bridge network. One output terminal II3 of the bridge network is connected to the grounded conductor 60 and the other output terminal II4 of the bridge network is connected by a conductor III to the sliding contact IOI which is disposed in engagement with the resistance 96.

The bridge network I05 is provided in order that the speed of the reversible motor I5 may be as great as possible during the rebalancing operation of the potentiometric network 4 without over-shooting of the balance point and consequent hunting taking place. It operation is explained in detail hereinafter.

The output circuit of the triode 16 is resistance-capacity coupled to the input circuits of the triodes I02 and I03 by means of a condenser IIS and a fixed resistance II1. In particular, the anode of the triode 15 is connected through the condenser Hi to both of the control electrodes of the triodes I02 and I03 and the said control electrodes are both connected through the resistance II? to the grounded conductor 60. Thus, the input circuits of the triodes I02 and I03 are connected in parallel. The resistance IIO of the bridge network I05 constitutes a biasing resistance for the input circuits of the triodes I02 and I 03 and is connected between the cathodes of the triodes I02 and I03 and the grounded conductor 60. The resistance II1 connected in the input circuits of the triodes I 02and I03 operates to maintain the control electrodes of the said triodes at the same potential as the negative terminal of resistance IIO when no voltage is induced in the transformer secondary winding 45, and upon the induction of a voltage in the transformer secondary winding 45, resistance I I1 permits the flow of grid current to thereby limit the extent to which the control electrodes of the triodes I02 spect to their associated cathodes. I

The motor I5 is preferably so constructed that the impedance of the winding 13 is the proper and I03 may go positive with re- 10 value to match the impedance of the anode circuits of the triodes I 02 and I 03 when the motor is operating in order to obtain th most efllcient the motor is so constructed that it has a high ratio of inductance to resistance, for example, of the order of 6 to 1 or from 8 to 1 at the frequency of the energizing current supplied to it. This provides efllcient operation and power during the runnin condition of the motor with the least amount of heating and also provides a. low impedance path which makes possible a desirable braking action.

The condenser 10 connected in the energizing circuit of the motor winding 12 is so selected with respect to the inductance of the latter as to provide a series resonant circuit having a unity power factor. By virtue of the series resonant circuit, the total impedance of the motor winding 12 is substantially equal to the resistance of the winding, and since this resistance is relatively low a large current flow through the motor winding 12 is made possible. This permits the attainment of maximum power and torque from the motor I5., In addition, the current flow through the motor winding 12 is in phase with the voltage of the alternating current supply mains 31 and 38 because of the series resonant circuit. The voltage across the motor winding 12, however. leads the current flow by substan tially because of the inductance of the winding 12.

Energizing current is supplied to the motor winding 13 from the transformer secondary winding 58 through the anode circuits of the triodes I02 and I03 through the circuits previously traced. The condenser II2 which is connected across the input terminals of the bridge network I05, one arm of which includes the motor winding 13, is so chosen as to provide a parallel resonant circuit with the motor winding 13 having substantially unity power factor. This parallel resonant circuit presents a relatively high external impedance and a relatively low local circuit impedance. To this end, the resistance H0 is desirably of small value. The relatively high external impedance is approximately the same as the impedance of the anode circuits of the triodes I02 and I03 and therefore provides eflicient operation. The relatively low or internal circuit impedance approximates the actual resistance of the motor winding 13, and since this resistance is relatively low the impedance of the local circuit is also relatively low.

, For the condition when the potentiometric network 4 is balanced, no voltage is induced in the transformer secondary winding 45, and hence, the potentials of the control electrodes of all of the triodes 14, 15, 15, I02 and I03 remain substantially constant. Under this condition of operation a pulse of unidirectional current flows through the anode circuit of the triode I02 to the motor winding 13 from the left hand section of the transformer secondary winding 58 during the first half cycle of the alternating voltage supply. In the second half cycle of the alternating voltage supply a pulse of unidirectional current flows through the anode circuit of the triode I03 to the motor winding 13 from the right hand section of'the transformer secondary winding 53. Since the control electrodes of the triodes I02 and I 03 are connected together and inasmuch as the potentials of these control electrodes remain substantially constant when the potentiometric network 4 is balanced, pulses of equal magnitude now in the anode circuits of operation. Preferably,

the triodes I02 and I03 during alternate half cycles of the alternatin voltage supplied by the transformer secondary winding 53. As a result when the potentiometric network 4 is balanced, pulsating unidirectional current of twice the frequency of the alternating supply current is impressed on the motor winding 13. When thus energized, the motor rotor II is not urged to rotation in either direction and remains stationary. Due to the relatively high direct current component of the current then flowing through the motor winding I3 the core structure of the motor I5 tends to become saturated whereby the inductive reactance of the motor winding 13 is relatively small. The condenser m in shunt to the motor winding I3 and the resistance IIo are so chosen that the condenser and motor winding then form a parallel resonant circuit. This saturation of the core structure of the motor I5 operates to exert a damping effect on the rotor 1| of the motor or in other words an effect tending to retard rotation of the rotor I I By virtue of this effect, if the rotor II has been rotating saturation of the motor core structure tends to stop the rotation.

Upon unbalance of the potentiometric measuring network 4 in one direction, the magnitude of the pulses of current flowing in the anode circuit of the triode I02 will be increased while the magnitude of the pulses of current flowing in the anode circuit of the triode I03 will be decreased. Consequently, the pulses of unidirectional current supplied to the motor winding I3 during the first half cycle will predominate over those supplied the motor winding I3 during the second half cycle. Such energization of the motor winding I3 operates to introduce therein an alternating component of current of the same frequency as that supplied by the alternating current supply mains 31 and 38. This alternating component of current may be either dephased 90 or 270 with respect to the alternating current flowing through motor winding I2 and produces an alternating magnetic field in the'motor core structure which reacts with the alternating magnetic field established by the motor winding I2 to produce a rotating field in one direction in the rotor. This rotating field rotates in one direction or the other depending upon the direction of potentiometric unbalance and effects actuation of the motor rotor H for rotation in a corresponding direction. When the motor winding 13 is so energized, the direct current component of current is decreased, and therefore, the saturation of the motor core structure is decreased with the result that the rotor damping effect is also reduced. Upon unbalance of the potentiometric measuring network 4 in the opposite direction, the conductivity of triode I03 will be rendered greater than that of triode I02 and as a result a rotating field will be established in the rotor II to urge the latter to rotation.

In order to obtain rapid operation of the reversible electrical motor I5 during the rebalancing operation of the potentiometric network 4 without the occurrence of overshooting and consequent hunting, the response of the motor I5 is correlated with the unbalancing and the rebalancing operations of the potentiometric network. This is accomplished by adjusting the contact I along the resistance 99 which is employed for the purpose of coupling the output circuit of the triode I to the input circuit of the triode 16. By moving the contact I00 in an upward direction the amplitude of signal on the control electrodes of triodes I02 and I03 is increased for any given signal induced in the transformer secondary winding 45 and by moving the contact I00 in a downward direction the amplitude of signal is decreased. This, therefore, adjusts the sensitivity of the electronic device 9 whereby the response of the reversible electrical motor I5 may be correlated exactly with the operation of the potentiometric network 4.

For many purposes such correlation and the damping action obtained as a result of the direct current component of current through the motor winding I3 is sufficient to prevent over-shooting and consequent hunting of the motor from occurring. There are other applications, however, wherein it is desired to effectuate the rebalancing operations of the potentiometric network 4 at a higher rate of speed and in those applications the damped operation obtained by means of the direct current component through the motor winding I3 and the above mentioned correlation is insufficient to prevent coasting and consequent hunting from occurring. In order that the speed of the reversible motor I5 may be increased so as to fulfill the conditions of such high'speed applications without over-shooting and consequent hunting occurring, means including the bridge network I05 have been provided in accordance with the present invention to insure that the motor speed is substantially proportional to the extent of unbalance of the potentiometric network 4. This result is obtained by deriving from the bridge network I05 a voltage of the same frequency as the voltage of the supply mains 31 and 38, the magnitude of which is a function of the motor speed and the phase of which is determined by the direction of rotation thereof, and by introducing this voltage into the electronic device 9 in opposition to the amplified quantity of the unbalanced voltage derived from the potentiometric network 4. With this arrangement, as the sliding contact 8 approaches its new balanced position, the unbalanced voltage of the otentiometric network 4 will decrease in value and if the motor speed is then such that it would ordinarily coast beyond the balanced position due to its inertia and the inertia of the associated rebalancing mechanism, the opposing voltage which is introduced into the circuit from the bridge network I05 will produce a temporary condition of rebalance before the true balanced position is reached to thereby anticipate the position at which the motor is deenergized for rotation. By proper choice of the circuit constants the opposing voltage derived from the bridge network I05 may be made greater than the amplified quantity of the potentiometric unbalanced voltage to produce a positive damping action which Will quickly check the motor speed before the balanced position is reached and gradually reduce it to zero as the potentiometric unbalance is reduced to zero. Such positive damping action results when the feedback voltage is greater than the amplifled quantity of the potentiometric unbalanced voltage because the motor I5 is then energized for rotation in the direction opposite to that in which it is actually rotating. Such energization is the equivalent of a powerful braking force. By virtue of the provision of the feedback bridge network I05, therefore, the speed of operation of the reversible motor I5 in effecting rebalance of the potentiometric network 4 may be made exceedingly great without the occurrence of overshooting and consequent hunting of the potenat least to a substantial extent and, if desired,

entirely, the eil'ect of the inductance of motor winding I3 so that when the motor rotor II is at rest the bridge network I 05 is substantially balanced, and hence, little or no unbalanced voltage useful for motor braking appears at the bridge output terminals H3 and H4. If desired, the value of condenser I09 ma be so chosen in relation to the other circuit components that the bridge network I05 is exactly balanced when the rotor II is stationary, but in order to obtain an optimum condition of stability in the balanced state, we have discovered that it is desirable for the bridge network I05 to be so adjusted as to be slightly out of balance, when the motor is at rest, in the proper sense that a small electromotive force of the same frequency as that of the amplified quantity of the potentiometric unbalanced electromotive force is fed back in opposition t the latter. It has been found that such arrangement substantially eliminates the tendency of the rotor II of the motor to chatter when the potentiometric network 4 is in its balanced condition and appreciably improves the stability of operation.

On rotation of the motor rotor ii an electromotive force of one phase or of opposite phase relatively to the voltage of the supply mains 31 and 38 and of the same frequency is produced at the bridge output terminals H3 and H4. The magnitude of this electromotive force is dependent upon and varies in accordance with the speed of rotation of the rotor II, and is impressed through conductors I I5 and 80 across a portion of resistance 95 in the input circuit of triode I5 depending upon the adjustment of contact IOI. The said electromotive force is so fed back to the resistance 96 as to oppose the fluctuating electromotive force produced across the latter as a result of potentiometric unbalance. The amount of the said electromotive force which is fed back may be adjusted b manipulation of contact IOI along resistance 96.

The precise manner in which an electromotive force is produced at the bridge output terminals I I3 and I I4 upon motor rotation is not now known to us, but it is believed to be a complex quantity created by two effects, one of which is due to transformer action between the motor windings I2 and I3 and the other of which is due to change in impedance of the winding I3. Both of these two eliects are additive, that is, the electromotive force created at the bridge output terminals H3 and H4 by one of said effects augments the electromotive force there produced by the other cfl'ect.

The transformer action effect will be first explained. Since the motor windings I2 and I3 are displaced 90 with respect to each other on the core structure of the motor, no lines of magnetic flux established by the winding I2 link any turns of the winding I3 when the rotor II is stationary, and hence, no electromotive force is then induced in winding II as a result of transformer action between windings I2 and I3. Upon rotation of the rotor-II. however, the magnetic flux established by the winding I2 is distorted whereupon some lines of said magneticiiux link the turns of winding I3 to induce an electromotive force in the latter of the same frequency as the voltage supplied by mains 31 and II. The induction of this electromotive force in winding 13 causes an electromotive force of the same frequency to appear at the bridge output terminals H3 and II4. The magnitude and the phase of this electromotive force relatively to the voltage of the supply mains 31 and 33 varies in accordance with the speed of rotation of rotor II and the direction of rotation thereof, respectively, inasmuch as the extent of distortion of the magnetic field of winding I2 is dependent upon the speed of rotation of rotor II and the direction of distortion is determined by the direction of rotor rotation.

The manner in which a component of electromotive force is produced at the bridge output ter-' minals I I3 and I I4 upon motor rotation by virtue of change in impedance of the motor winding I3 occasioned by such rotation will now be explained. As has been explained previously, the constants of the bridge network I05 are so chosen that at the frequency of the voltage supplied by mains 31 and 33 the bridge network I05 will be either exactly balanced or slightly out of balance where greater stability is desired. The value of condenser I03 is then effective to either entirely, or almost entirely, cancel out the inductive effect of motor winding I3. Upon motor rotation, however, the effective coupling between the motor windings I2 and I3 is changed and this effective change in coupling produces an apparent change in inductance and also in resistance of winding I3. At such new apparent value of inductance and resistance of winding I3 the condenser I03 is not operative to balance the bridge network to the same extent as when the motor is at rest, and hence. the bridge network I05 becomes unbalanced. The extent of bridge network unbalance which is efiected in this manner varies in accordance with the speed of rotation of the rotor II since the apparent change in impedance of winding I3 is dependent upon the speed of rotation of rotor II. The change in inductance of winding I3 with motor rotation is always in the same direction regardless of the direction of motor rotation, and hence, the bridge network is unbalanced in the samedirection irrespective of the direction of motor rotation.

While the bridge network I 05 is always unbalanced in the same direction regardless of the direction of motor rotation, such unbalance nevertheless is operative to cause an electromotive force in phase with the voltage of the supply mains 31 and 38 to appear at the bridge output terminals I I3 and H4 upon rotation of the motor in one direction and to produce an electromotive force out of phase with the voltage of the supply mains 31 and 38 at the bridge output terminals upon rotation of the motor in the opposite direction. Such phase shift upon reversal in the direction of motor rotation is produced because pulsating current in phase with the voltage of the supply mains H3 and H4 is impressed on the bridge input terminals I04 and I06 when the potentiometric network 4 is unbalanced in one direction, and pulsating current of opposite phase is impressed on the bridge input terminals when the potentiometric network 4 is unbalanced in the opposite direction. In the first case, the niotor is energized for rotation in one direction, and in the second case, the motor is energized for rotation in the opposite direction. Consequently, the same change in impedance of motor winding I3 produces an electromotive force of one phase at the bridge output terminals 3 and H4 when the motor is energized for rotation in one direction and produces an electromotive force of opposite phase at the bridgeoutput terminals when the motor is energized for rotation in the opposite direction.

As the motor rotor II gains speed during a rebalancing operation of the potentiometric network I, the unbalance of the potentiometric network is partially compensated for by the complex quantity derived from the bridge network I05 to thereby establish a new, although temporary, balanced position of sliding contact 8 along the slidewire resistance 6. When the contact 8 subsequently moves into that temporary balanced position, the motor I5 will be deenergized for rotation and will slow down. Accordingly, as the slidewire contact 8 approaches its new balanced position, the energization of motor I5 will be interrupted before the true balanced position is reached and will gradually ease into its new balanced position without coasting beyond. During such a rebalancing operation, the opposing voltage which is introduced into the electronic device 9 from the bridge network I05 may be made appreciably greater than the unbalanced voltage obtained from the potentiometric network 4 and, as a result, the motor l5 may actually be energized for rotation in the reverse direction from that in which it is rotating. Such operation produces a. positive damping or braking action on the motor rotor which will quickly stop its rotation and positively prevent coasting beyond the true balanced position, and hence, makes possible rebalancing of the potentiometric network 4 by the reversible motor [5 and the associated rebalancing mechanism at a very high' rate of speed without the occurrence of overshooting and consequent hunting.

It is noted that the bridge network I05 illustrated in Figs. 1 and 2 is of a type which may be exactly balanced only at one frequency of the energizing electromotive force impressed on its input terminals. In practice, the bridge components are so chosen that the bridge network is balanced when fluctuating voltage of the frequency of the supply lines 31 and 38 is impressed on its input terminals I04- and I06. For reasons of stability, as previously noted, the constants are so chosen that some slight output voltage is derived from the bridge network when the motor is at rest. For all other frequencies of applied voltages, the bridge network is not balanced, and hence, voltage components having such other frequencies will appear at the output terminals H3 and I of the bridge.

With the arrangement of Figs. 1 and 2 the voltage impressed on the input terminals I04 and I06 from the motor drive stage including tube I4 and transformer secondary winding 58 will include a component of twice the frequency of the voltage supplied by mains 31 and 38, and accordingly, an electromotive force of this higher fre: quency will be produced at the bridge output terminals H3 and Ill and will be impressed through the feedback conductors 60 and H5 across the resistance 96 in the input circuit of triode I5 even when the rotor II of motor I5 is at rest. This higher frequency of electromotive force is not effective to actuate motor I5 for rotation, however, since the motor drive stage is of a frequency discriminating type and will not respond to voltages of twice the frequency of that supplied by the supply mains 31 and 38 for energizing motor I5 to rotation.

Although the presence of such a double frequency component of electromotive force at the bridge output terminals I04 and I06 is not effective to cause motor rotation, it may nevertheless be desirable in some instances to eliminate such double frequency component because of the loading effect of the latter upon the electronic device 9, and in such applications we prefer toemploy the embodiments of our invention illustrated in Figs. 3-6.

The modification of Fig. 3 is generally similar to that of Figs. 1 and 2 both in appearance and operation but differs therefrom in that a bridge network I05a is utilized'in lieu of the bridge network I 05. The bridge network I05a includes the motor winding I3 in one arm, fixed resistances H0 and III in two other arms, and a condenser I09 shunted by a resistance I08 in the remaining arm. Such a bridge network is known generally in the art as a Maxwell bridge. In a bridge of this type the state of balance, according to theory, is not afiected by variation in frequency of the voltage impressed on its input terminals. Accordingly, the bridge network I05a is characterized in that when the motor rotor II is stationary little or no undesired components of voltage which may load or otherwise adversely affect the electronic device 9 are produced at the bridge output terminals.

In Fig. 4, we have illustrated'another modification of the arrangement illustrated in Figs. 1 and 2 which may be utilized for obtaining rebalancing of the potentiometric network 4 at high speed without coasting and hunting occurring and in which voltage components of undesired frequency in the feedback circuit have been eliminated. The arrangement of Fig. 4 differs from that shown in Figs. 1 and 2 in that the condenser I09 of Figs. 1 and 2 has been eliminated and an inductance H8 is utilized instead for balancing the bridge network I05b when the motor rotor is stationary. As shown in Fig. 4, the bridge network I05b includes the motor winding I3 in one arm and the inductance H8 and a fixed resistance H9 in series in an adjacent arm. The remaining arms are comprised of a fixed resistance III and a variable resistance I20, respectively. The operation of the bridge network I05b in producing an electromotive force of one phase or of opposite phase upon motor rotation in one direction or the other and of a magnitude depending upon the speed of motor rotation is essentially the same as that of the bridge-network I05 of Figs. 1 and 2.

The arrangement of Fig. 4 difiers further from that of Figs. 1 and 2 in that the output potential from the bridge network I05b is impressed across the terminals of a fixed resistance I2I and a variable part of the potential drop produced across the latter is impressed 0n the input circuit of the triode I5 depending upon the adjustment of a sliding contact I22 along the resistance I2I. By adjusting the contact I22 along resistance I2 I, the magnitude of the opposing voltage introduced into the input circuit of triode I5 from the bridge network I05b may be varied as required to exactly compensate for the hunting tendency of the reversible motor I5 during the rebalancing operation. With this arrangement the resistance connected in circuit with the bridge output terminals remains constant regardless of the position of contact I22 along resistance I2I. This is a desir able feature not obtained in the Figs. 1 and 2 form of our invention.

By proper choice of the components of bridge network I851), undesired components of voltage of frequencies other than that of the supply mains 31 and 38 may be prevented from appearing at the output terminals H3 and H4 regardless of whether the motor I5 is rotating or is at rest. To this end, the saturation characteristics of the fixed inductance II8 are desirably so related to the saturation characteristics of the motorewinding I8 that the bridge network I051) will be balanced, whether voltage of the frequency of the supply mains 31 and 88 is impressed on the input terminals I84 and I08 from the motor drive tube It and transformer secondary winding 58, or

I whether voltage of twice that frequency is impressed on the input terminals from said motor drive tube and transformer winding, as the energ-ization of motor I5 is increased from zero speed energization to full speed energization.

In Fig. 5, we have illustrated a modification of the arrangement of Fig. 4 wherein the damping signal derived from the bridge network I65b is introduced into a later amplifying stage of the electronic device 8. To this end, the output voltage of the bridge network I65b is impressed on the input circuit of the triode 16, instea of on the input circuit of the triode I5 as in the arrangements of Figs. 1 and 2, through a transformer I23. The transformer I23 is provided with a primary winding I24 which is connected to the terminals I I 3 and I ll of the bridge network H152) and is provided with a secondary winding I25 which is connected to the terminals of a resistance I26. One terminal of the resistance I26 is connected to the rounded conductor and a contact I21 which is in slidable engagement with the resistance I26 is connected to the lower terminal of the resistance 98.

In this modification, a variable part of the potential drop produced across the resistance I26. depending upon the adjustment of the contact I21, is impressed on the input circuit of the triode 16 in opposition to the signal impressed on the input circuit of the triode I6 from the output circuit of the triode 15. The transformer I23 preferably is a step-up transformer. This factor makes it possible to feed back the opposing electromotive force from the bridge network I85b to the later stage 16 of the electronic device 9 while still obtaining the same degree of damping.

In Fig. 6, we have illustrated another modification of our invention in which a bridge network I050 having fixed resistances III) and III in two arms and a variable resistance I20 in a third arm is utilized in lieu of the bridge network I05. The fourth arm of the bridge network I85c is comprised of two branches in one of which the motor winding I3 is connected and in the other of which a resistance I 28 and a condenser I29 are connected in series. The resistance I28 preferably has a value approximately equal to the resistance of the winding I3 and the condenser I29 is so chosen that the resistance of the winding I3 or of resistance I28 is equal to the square root of the quantity comprising the inductance of motor winding 13 divided by the capacitance of the condenser I29. Thus, if the resistance of the winding I3 is R1, the resistance of the resistance I28 is Rs, the inductance of the winding I3 is L, and

18 the capacitance of the condenser I29 is C, the relationship may be stated as follows:

With the components I3, I28 and I29 so chosen, it may be proved mathematically that the total impedance of the bridge arm including these elements is resistive in character and has the value of either the resistance I28 or of the winding I8. The bridge network I Die, therefore, essentially includes resistive elements in each of its arms, and therefore, the electromotive force produced between the output terminals H3 and III will be exactly in phase with or out of phase with the voltage of the supply mains 31 and 38. Accordingly, the voltage fed back in opposition to the controlling signal in the electronic device 8 will be exactly 180 out of phase therewith. It is noted further that the character of the bridge network I650 of being essentially a resistive bridge is not altered by change in frequency of the energizing current impressed on the input terminals I64 and I66. Hence, the bridge network III5c remains balanced regardless of the frequency of the voltage impressed on its input terminals.

The conversion type potentiometer and the electronic amplifying and motor drive rebalancing means therefor disclosed in this application and not claimed herein is disclosed and is being claimed in the aforementioned Wills Patent No. 2,423,540.

While in accordance with the provisions of the statutes, we have illustrated and described the best forms of embodiment of our invention now known to us, it will be apparent to those skilled in the art that changes may be made in the forms of the apparatus disclosed without departing from the spirit of our invention as set forth in the appended claims, and that in some cases certain features of our invention may be used to advantage without a corresponding use of other features.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, a device adjustable to reduce said unbalanced electromotive force, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, a resistance connected in said output circuit to supply biasing grid potentials to said electronic devices, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a, bridge network including said resistance in one arm and said second mentioned winding in another arm to derive an alternating electromotive force proportional to the motor speed, and means to oppose said derived electromotive force to said unbalanced electromotive force.

2. A system comprising an electrical network,

means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, a device adjustable to reduce said unbalanced electromotive force, an alternating current supply circuit, a pair of grid controlled electronic devices having outputcircults connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, a resistance connected in said output circuit to supply biasing grid potentials to said electronic devices, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic. devices, a normally balanced bridge network including said resistance in one arm and said second mentioned winding in another arm and adapted to be unbalanced upon motor rotation to derive an alternating electro motive force proportional to the motor speed, said bridge network including a reactance in one of the arms thereof to compensate for the reactance of said second mentioned motor winding for balancing said bridge when said motor is not rotating, and means to oppose said derived electromotive force to said unbalanced electromotive force.

3. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, a device adjustable to reduce said unbalanced electromotive force, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance, with the phase of said unbalanced electromotive force, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including said second mentioned winding in one arm and adapted to be unbalanced upon motor rotation to derive an alternating electromotive force proportional to the motor speed, said bridge network including a condenser in one of the arms thereof to compensate for the reactance of said second mentioned motor winding for balancing said bridge when said motor is not rotating, and means to oppose said derived electromotive force to said unbalanced electromotive force. a

4. The combination of claim 3 wherein said condenser is included in an arm of said bridge network different from that in which said second mentioned winding is included.

cuits connected in opposite phase relation to said alternating current circuit and havin a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances only in three arms and said second mentioned winding in the fourth arm and adapted to be unbalanced upon motor rotation to derive an alternating electromotive force proportional to the motor speed, said bridge network including a condenser and a resistance in shunt to said second mentioned winding to compensate for the reactance of said second mentioned motor winding for balancing said bridge when said motor is not rotating, and means to oppose said derived electromotive force to said unbalanced electromotive force.

6. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, a device adjustable to reduce said unbalanced electromotive force, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said grid controlled electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in series in the fourth arm to derive an alternating electromotiveforce proportional to the motor speed, and means to oppose said derived electromotive force to said unbalanced electromotive force.

7. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, a device adjustable to reduce said unbalanced electromotive force, an alternating current supply circuit, a. pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said grid controlled electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, an alternating current motorto adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in parallel in the fourth arm to derive an alternating electromotive iorce proportional to the motor speed, and means to oppose said derived electromotive force to said unbalanced electromotive force.

8. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, a device adjustable to reduce said unbalanced electromotive force, an alternating current supply circuit, an electronic amplifier having a plurality of stages and having an input circuit to which said electromotive force is applied, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, an output circuit for said electronic amplifier to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in the fourth arm to derive an alternating electromotive force proportional to the motor speed, and means to oppose said derived electromotive force to the amplified quantity of said unbalanced electromotive force at an intermediate stag of said electronic amplifier.

9. A system comprising an electrical network, means to produce an alternating signal of one phase or of opposite phase in said network, a device adjustable to reduce said alternating signal, an alternating current supply circuit, an electronic amplifier having a plurality of stages and having an input circuit to which said electromotive force is applied, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, an output circuit for said electronic amplifier to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said alternating signal, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in series in the fourth arm to derive an alternating signal of magnitude proportional to the motor speed and of the same frequency as said first mentioned alternating signal, and means to oppose said derived alternating signal to the amplified quantity of said first mentioned alternating signal at an intermediate stage of said electronic amplifier.

10. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, a device adjustable to reduce said unbalanced electromotive force, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alter-' nating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, an alternating current motor to adjust said first mentioned device having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in three arms and said second mentioned motor winding in the fourth arm shunted by a resistance and a condenser connected in series to derive an alternating electromotive force proportional to the motor speed, and means to oppose said derived electromotive i'orce to said unbalanced electromotive force.

11. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, a reversible alternating current motor having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances only in three arms and said second mentioned winding in the fourth arm and adapted to be unbalanced upon motor rotation to derive an alternating electromotive force proportional to the motor speed, said bridge network including a condenser and a resistance in shunt to said second mentioned winding to compensate for the reactance of said second mentioned motor winding for balancing said bridge when said motor is not rotating, and means to oppose said derived electromotive force to said unbalanced electromotive force.

12. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said grid controlled devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, a reversible alternating current motor having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in series in the fourth arm to derive an alternating electromotive force proportional to the motor speed, and means to oppose said derived electromotive force to said unbalanced electromotive force.

13. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said grid controlled electronic devices to selectively render one or said devices more conductive than the other in accordance with the phase or said unbalanced electromotive force, a reversible alternating current motor having a winding connected to said alternating current supply circuit and a winding connected to the output circuit or said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in parallel in the fourth arm to derive an alternating electromotive force proportional to the motor speed and means to oppose said derived electromotive force to said unbalanced electromotive force.

14. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, an alternating current supply circuit, an electronic amplifier having a plurality of stagesand having an input circuit to which saidelectromotive force is applied, a pair of grid controlled electronic devices having output circuits connected in opposite phase relatiorl to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, an output circuit for said electronic amplifier to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, a reversible alternating current motor having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in the fourth arm to derive an alternating electromotive force proportional to the motor speed, and means to oppose said derived electromotive force to the amplified quantity of said unbalanced electromotive force at an intermediate stage of said electronic amplifier.

15. A system comprising an electrical network, means to produce an alternating signal or one phase or or opposite phase in said network, an alternating current supply circuit, an electronic amplifier having a plurality of stages and having an input circuit to which said electromotive force is applied, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, an output circuit for said electronic amplifier to supply grid potentials to said electronic devices to selectively render one or said devices more conductive than the other in accordance with the phase of said alternating signal, a reversible alternating current motor having a winding connected to said alternating current supply circuit and a winding connected to the output circuit of said electronic devices, a normally balanced bridge network including resistances in diametrically opposed arms, said second mentioned motor winding in a third arm and a resistance and a condenser in series in the fourth arm to derive an alternating signal of magnitude proportional to the motor speed and of the same frequency as said first mentioned alternating signal, and means to oppose said derived alternating signal to the amplified quantity of said first mentioned alternating signal at an intermediate stage of said electronic amplifier.

16. A system comprising an electrical network, means to produce an unbalanced alternating electromotive force of one phase or of opposite phase in said network, an alternating current supply circuit, a pair of grid controlled electronic devices having output circuits connected in opposite phase relation to said alternating current circuit and having a common output circuit to which said electronic devices are connected in parallel relation, means to supply grid potentials to said electronic devices to selectively render one of said devices more conductive than the other in accordance with the phase of said unbalanced electromotive force, a reversible alternating current motor having a winding connected to said alternating current supply circuit and a winding connected to the output circuit or said electronic devices, a normally balanced bridge network including resistances in three arms and said second mentioned motor winding in the fourth arm shunted by a resistance and a condenser connected in series to derive an alternating electromotive force proportional to the motor speed, and means to oppose said derived electromotive force to said unbalanced electromotive force. 3

THOMAS R. HARRISON. LLOYD B. CHERRY.

REFERENCES CITED The following references are of record in the file of this patent:

Jones May 22, 1945 

